Atom probes are analytical instruments that analyze the atomic-level composition of materials by field evaporation of atoms and small molecules from a specimen, and measuring their time of flight (TOF) from the specimen to a detector some distance away. See, for example, U.S. Pat. Nos. 5,061,850, 5,440,124 and 6,576,900 to Kelly et al.; International Publications WO 99/14793 and WO2004/111604; and Kelly et al., Ultramicroscopy 62:29-42 (1996).
In a typical atom probe, the specimen is in the form of a sharp tip (often having a tip radius of ˜50 nm), and is held at a semi-static standing voltage that is below that necessary to cause field evaporation of the atoms at the tip of the specimen. A counter electrode, which usually has an aperture therein, is spaced about or at a slight distance from the specimen tip, with the specimen tip pointing through the aperture. A pulsed (usually negative) voltage is applied to the counter electrode, and/or a pulsed (usually positive) voltage is applied to the specimen, with sufficient magnitude to ionize the specimen tip, preferably a single atom at a time. Ionization usually does not occur with every pulse, and rather occurs once per several pulses (often with one ionization event for every 10-100 pulses). The amplitude of this pulse, called the “ionization pulse,” is typically 10% to 25% of the standing voltage.
During the initial stages of analysis the specimen tip rapidly adopts a nominally hemispherical end form, since any atom that is more “exposed” to the ionizing field will be preferentially evaporated. The hemispherical end form of the tip creates an electric field that is nearly radial, and consequently when a specimen atom is ionized, it flies radially away from the specimen, through the aperture of any counter electrode, and toward a 2-dimensional (2D) particle detector (generally located 10-100 mm away from the specimen tip). The position at which the ion impacts the detector is measured, and this impact position is uniquely correlated with the ion's original position on the specimen surface. In this manner the specimen tip (of for example 50 nm size) is effectively projected onto the detector (of for example 40-100 mm size), yielding roughly a million-fold factor of magnification.
Apart from monitoring the ion impact position, time of flight (TOF) mass spectroscopy is performed on the evaporated ions by measuring the time between the application of the ionization pulse (which roughly indicates the time of ion departure from the specimen) and the subsequent ion impact at the detector. The TOF measurement can be directly correlated to the mass to charge ratio (MTC) of the ion, which in turn can allow identification of the ionized atomic (or molecular) species. Thus, by utilizing the magnified “image” of the specimen and the elemental identification provided by the TOF mass spectroscopy, a 3-dimensional atom map of the specimen can be created.
One of the inherent limitations of atom probes is that for a given MTC ratio (i.e., for a particular ionized species), a range of TOF values can be measured. This inherent spread in the TOF measurement limits the ability of atom probe techniques to distinguish between atomic (or molecular) species of nearly the same MTC ratio. In other words, the peaks in the TOF histogram of two different species may overlap, making it difficult to assign a specific MTC ratio to each species, and thereby making it difficult to identify the ions that are recorded in the overlapped region. Thus, there is a limit to the mass resolution (ionic species identification) capability of an atom probe.
A second order effect of the finite mass resolution is decreased sensitivity to low concentration species. All atom probes record spurious events—for example, ionization events that occur independent of ionization pulses, “rogue” species in the atom probe which impact the detector, etc.—that contribute to a finite noise floor. In order for a given species to be definitively identified, it must be present in quantities that are statistically significant compared with the noise floor. The smaller the range in measured TOF, the more quickly a valid signal will emerge from the noise.
One factor reducing the mass resolution in all atom probes that utilize an ionization pulse to initiate field evaporation of specimen ions is the (relatively small) uncertainty in the time of ion departure upon application of the ionization pulse, and the corresponding energy (velocity) that is imparted to the departing ion. This phenomenon is illustrated in FIG. 1, which schematically illustrates an exemplary plot (depicted as voltage versus time) of an ionization pulse at 100. (While the ionization pulse 100 is typically negative and delivered to a counter electrode, it is shown positive in FIG. 1 for clarity.) The rate at which ions field evaporate from a surface has been shown experimentally (in accordance with theory) to be exponentially dependent upon field strength, which is in turn linearly related to the applied voltage.
As a result of the exponential nature of field evaporation, nearly all specimen ion evaporation events occur very near the peak voltage of the ionization pulse 100, with the range Δt in FIG. 1 illustrating the time interval over which most ionization occurs. The exact time at which any given atom or molecule is ionized during the ionization pulse 100 is described by the probabilistic distribution shown schematically at 102. The exact width of the distribution 102 varies with many experimental parameters, such as specimen material and temperature. Nevertheless, in all cases, the result is an uncertainty Δt in the exact ionization time of any given atom or molecule relative to the time to corresponding to the peak voltage of the ionization pulse 100.
After being ionized, the atoms or molecules are accelerated by the electric field caused by the combination of the standing voltage and the ionization pulse voltage until the ions enter a relatively field-free region just inside the aperture of the counter electrode (if one is present). An atom or molecule that is ionized before the peak of the ionization pulse experiences an increasing field as it is accelerating away from the specimen and will therefore acquire more energy (i.e. velocity) as compared an atom or molecule that is ionized at the same voltage, but after the peak. Thus, there is a range of ion departure velocities, with most ions having velocities varying in the range Δv shown in FIG. 1 (which schematically illustrates the velocity distribution of ions at 104 in accordance with their time of ionization).
Therefore, any given atom or molecule that is ionized in an atom probe will have an uncertainty Δt in the exact instant of ionization, and in the exact velocity (Δv) it acquires during and after the ionization process. As a given ion type traverses the distance from the specimen to the detector, the combination of Δt and Δv gives rise to a spread in the measured time of flight. This variation limits the ability to resolve species that have nearly identical MTC ratios. By varying the design of the atom probe, the exact form of FIG. 1 can be altered significantly—for example, the ions leaving early during the ionization pulse may be the slowest—but for a given design the variation in velocity versus the exact instant of ionization will be systematic, and therefore (at least theoretically) correctable.
In practice, it is the velocity distribution Δv that creates the majority of the uncertainty in measured TOF, and consequently limits mass resolution in conventional atom probes. Traditionally, the atom probe and mass spectrometry communities refer to the velocity distribution Δv inherent in atom probes as the “energy deficit,” and the process of reducing the spread in the velocity distribution is called “energy compensation”. (Additionally, it should be understood that “velocity distribution” usually refers to the distribution of velocities for a particular species of ions evaporated from a specimen, not to the far wider distribution of velocities across all species.) An atom probe without any form of energy compensation will typically possess a mass resolution of 1 part in 80-200 as measured by the full-width at half-maximum (FWHM) of a given mass peak in the spectrum. A variety of energy compensation schemes have been employed, including:
(1) Reflectrons. A reflectron is essentially an electrostatic mirror. Ions from the specimen are directed into the reflectron, where they stopped by a uniform decelerating electrostatic field. The same field then accelerates the ion back out of the reflectron at a small angle to the incident beam. Faster ions penetrate more deeply into the reflectron than slower ions, and therefore spend more time in the reflectron. If the distances between the specimen, reflectron, and detector are carefully chosen, the spread in measured TOP times can be reduced. Mass resolutions of 1 part in 800 (FWHM) have been reported for atom probes with reflectrons. The main disadvantage of reflectrons is that only a small range in the incident angle of incoming ions is properly reflected, limiting the use of the reflectron to 1-D atom probes, and to 3-D atom probes that have a relatively small angle of view.
(2) Post Acceleration. In post acceleration, after the initial ionization event, all of the ions are accelerated to a significantly higher velocity by a constant voltage, known as a post-accelerating voltage, for the remainder of the flight distance to the detector. By increasing the velocity of the ions by a constant voltage, the fraction of the velocity due to the ionization pulse voltage—which is the source of the velocity variation—is minimized, and mass resolution is increased. The main disadvantages to this approach are that the amount of mass resolution improvement is asymptotically limited to a modest amount for reasonable instrument geometries and post acceleration voltages. Experimental results employing this technique suggest that mass resolutions of 1:400 to 1:600 (FWHM) are possible.
(3) 163° Poschenrieder Energy Compensating lens. This technique employs a semicircular ion flight path of 163° created by electrostatic fields to compensate for the differences in ion velocities. A faster ion traverses the semicircular flight path with a slightly larger radius than that of a slower ion, and as a result, it has a longer flight length. If the proper dimensions are calculated—the 163° angle is the result of analytical calculations—the different flight paths/lengths of the ions result in the ions having the same flight times to the detector. Mass resolutions of 1:5000 (FWHM) have been achieved with this technique. The main limitation of this technique is that it destroys information related to ion position, and is therefore limited to 1D atom probes where knowledge of the original positions of the ions on the specimen is not needed.
(4) Ion Deceleration Via a Counter Electrode. This technique is schematically depicted in FIG. 2A, wherein a specimen 200 is shown in an atom probe chamber 202 spaced from a detector 204, and with the specimen 200 being connected to a source 206 of standing voltage. Departing ions (illustrated by flight cone 208) pass in turn through a first counter electrode 210 connected to an ionization pulser 212, and then through a second counter electrode 214 which is well connected to ground 216 (or to some other constant potential equal to the non-pulsed potential of the first counter electrode 210, as depicted in FIG. 2C). When the first counter electrode 210 is pulsed by the pulser 212 to ionize atoms on the specimen 200, the ions traveling to the second counter electrode 214 are all slowed to approximately the same velocity (one corresponding to the non-pulsed potential of the electrodes 210 and 214). This results in a reduction in the spread of the velocity distribution caused by the duration and magnitude of the ionization pulse. Mass resolutions of approximately 1:350 (FWHM) have been reported with this technique. The main limitation of this technique is the modest increase in mass resolution. FIG. 2B illustrates the analogous circuit for FIG. 2A, wherein the inherent capacitances 218 and 220 between the first counter electrode 210 and the specimen 200, and between the second counter electrode 214 and both of the first counter electrode 210 and the specimen 200, are depicted; these capacitances will be relevant to later discussion.
It would therefore be useful to have available some means for attaining better mass resolution in atom probes while reducing or eliminating the difficulties involved with the prior mass resolution enhancement techniques.